Yes you can, I was thinking of doing that myself.
I'd grab a flyer in front of the arts students centre, which shows how your study plan would work.
Disadvantage:
* Most people will start the diploma in second year, however have completed their language units in first year. This is to ensure that the first year language studies can count towards both the bachelor's breadth requirements and towards the diplang (also known as cross-crediting). In your situation, you would be studying your language units in years 2, 3 and 4 and only those units will count towards the diploma, i.e. you would need to still pick breadth subjects (4 minimum).
The fourth year is to ensure that you get enough credit points to graduate. I.e. during your third year you probably won't have completed enough level 3 subjects from your bachelor of arts so you will take some of those subjects in fourth year.
In the meantime though, grab a few information booklets of the diplang in front of the arts student centre